Porous materials, especially solid porous materials, are widely used in a variety of industrial contexts. For instance, in the field of host-guest chemistry (a particular branch of supramolecular chemistry) many porous materials can serve as “host” components for receiving (sometimes selectively) and reversibly retaining (through non-covalent binding) guest components within the host's pores. Host components may typically comprise compounds such as organic macrocycles, organic cages, covalent organic frameworks (COFs), zeolites and metal organic frameworks (MOFs). Some common host components include inter alia cryptands, crown ethers, cucurbiturils, cyclodextrins, calixarenes, cyclotriveratrylenes, cryptophanes, carcerands, zeolites, porphyrins, pillararenes, metallacrowns, and foldamers.
Of particular interest to the inventors are porous organic solids, such as organic macrocycles and cages, which can be especially useful as adsorbents, for instance for radioisotope pollutants,7 as molecular additives in organic-organic mixed-matrix membranes,8 as shape-selective chromatography phases,9 and as materials for molecular sensing.10 
Schiff-base chemistry is one of the most versatile methods for the construction of organic macrocycles and cages.1 The reversibility of the imine bond forming reaction gives a route to thermodynamically equilibrated products. This has been used to produce crystalline porous organic solids, such as covalent organic frameworks (COFs)2 and porous molecular organic cages.3 In particular, a series of porous, shape-persistent imine cages has been reported,4 and surface areas as high as 3758 m2 g−1 have been attained,5 thus rivalling extended frameworks such as MOFs.6 
However, the reversibility of imine chemistry, which permits equilibrium products to form, can also cause problems of chemical instability, thus limiting its wider application. Imines are prone to hydrolysis and can decompose upon exposure to atmospheric moisture, although there are some exceptions to this.2,11 Imines are even more prone to hydrolysis in acidic or basic environments. A straightforward way to make an imine more chemically stable is to reduce it to the corresponding amine. In addition to enhancing chemical stability, amine cages are also readily functionalized12 and can provide binding sites for guests such as CO2.13 However, while many amine cages and macrocycles have been reported via imine reduction, the resulting porous structures are often inherently unstable owing to the additional flexibility introduced to the cage molecule following imine reduction—i.e. the internal cavities or pores of such cage molecules can be prone to collapse, thereby prematurely nullifying their utility.
By way of example, to illustrate these pore-collapsing problems, the inventors previously reported a [4+6] amine cage formed by reduction of the equivalent imine cage, (designated CCI), but crystallization attempts yielded only amorphous, non-porous solids for the amine derivative.14 Zhang et al. reported a series of [2+3] amine cages but these, too, collapsed in the absence of solvent, and a very low level of porosity (but a high CO2/N2 selectivity) was observed.15 Mastalerz et al. reduced their [4+6] salicylbisimine cages to the corresponding amines, but this also resulted in a collapse of the cage, the loss of the intrinsic cage cavity, and a dramatic decrease in porosity.10,16 These examples all demonstrate that the increased flexibility of saturated amine bonds with respect to unsaturated imine bonds causes loss of shape-persistence in the molecule, even when the parent imine cage is shape-persistent and porous.
It is therefore an object of the present invention to solve at least one problem of the prior art.
Another object of the invention is to provide porous materials which have a relatively high degree of chemical stability across a range of conditions (e.g. stable to hydrolysis at various pHs, stable across a wide temperature range, and/or solvent stable) and a relatively high degree of physical stability (e.g. from a shape-persistence and porosity retention perspective).